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-   -   Hafler DH-200 Bias Problem (http://www.diyaudio.com/forums/solid-state/152916-hafler-dh-200-bias-problem.html)

Pyre 4th October 2009 02:49 PM

Hafler DH-200 Bias Problem
 
Hello,

I have a Hafler DH-200 that I am fixing for a friend. Its a factory mono block by the looks of it. The one side of the amp was heating up and blowing the fuse. I figured just about everything on that side needed to be replaced so I bought a set of MOSFETs and a kit with all the transistors and diodes. I tested everything on the board and replaced any resistor that was out of spec. I also replaced the two 100uf caps. I did not have any non polarized caps so I was unable to replace the other two. My problem is that after all this the amp still has the problem. I tested the bias and turned down all the way its 700ma+ which I cant imagine is good for anything and probably what cooked everything in the first place. What would cause the bias to be so high? Something must be blown but I cant figure out what. Any help would be greatly appreciated.

freddymac406 4th October 2009 06:02 PM

Transister Q9 is most likely at fault. It is a 2N2222.
Fred

Pyre 15th October 2009 12:35 PM

Thank you Fred, You were completely correct. I did not think so at first as I replaced it with a brand new 2N2222 that came with the transistor kit I bought. That is why I was so confused. I replaced everything on that board that did not test within spec. When I was at the parts store yesterday I bought a 10 bag of PN2222A's and soldered one in when I got home. Put it all together and it biased right up. Not sure why that 2N2222 did not work before but the amp works now and I can get some sleep at night and give them back to my friend.

Any idea why someone would have replaced all the grid stop resistors R40-R43 from 220R to 470R? Is it so it can handle more bias without oscillating?

nigelwright7557 15th October 2009 06:42 PM

One trick I learned with bias circuits is to put a zener across them to catch out of range voltages can that damage things.

All my amp designs have used this and not had an output stage pop yet.

freddymac406 15th October 2009 08:35 PM

You must be a tube guy. Grid stopping resisters ?
To answer your question they should be 470 Ohm but may have changed during production run. Also to improve the sound put a 20K resister across c9 on pc-6,
This will lower open loop gain and flatten responce so the -3 point will be 20K.
This lowers TIM distortion. I was a Service Tech @ Hafler in the early 80s when they where located in NJ.
Fred


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